Oil well pipe and casing are examples of tubular products used in production wells that are subjected to particularly stringent operating conditions. They must, for example, withstand extremely high mechanical loads when connected together in a long string, and at the same time be substantially unaffected by both internal and external pressures and corrosive environments. The extremely high pressures encountered, for example, can cause differential deformation of the threaded pin end of a pipe relative to the receiving collar if the pressurized gases or liquids penetrate between the threaded regions, sometimes causing decoupling of the string. One obvious response by those in the art to these conditions has been to employ special coupling arrangements, based on high strength sections and square or rectangular threads, such as shown in U.S. Pat. Nos. 4,009,893, 4,154,466, 4,209,193 and 4,253,687.
The practical state of the art is shown in a book widely used in the oil industry entitled "Tubular Connection Data", published by Weatherford/Lamb, a Weatherford International Company, 2nd edition, copyright 1978. This book depicts the great majority of couplings that are currently in field use today, including standardized American Petroleum Institute (A.P.I.) "8 round" and "buttress" tubing, and a number of specialized couplings using internal seals in the threaded region, corrosion barriers and the like.
While the special box (collar) and pin (pipe or casing) couplings are of theoretical benefit, they are of far less practical utility because they are nonstandard and expensive. There is today a vast inventory of A.P.I. pipe of 8 round and buttress type in different grades and weights which must be kept in use for obvious economic reasons. Equally obviously, the procedures used at the pipe rack and on the rig floor should involve a minimum number of conventional steps, and be quick, convenient and inexpensive to implement. It is particularly desirable to expand the range of field conditions under which A.P.I. pipe can be used while reducing the number of circumstances under which special equipment or instrumentation is needed. For more critical situations, equipment can be employed which counts turns, monitors torque or measures bearing pressures during makeup. Measurement of torque is not an accurate guide to engagement, because friction factors, thread profile and pitch, thread smoothness and lubricity all have an effect on the reading. When makeup is complete, instrumentation systems can be used to test the integrity of the seal with gas or liquid. Using both categories of equipment together is extremely costly but even where used does not permit expansion of the role of A.P.I. tubing. In a joint that is very tightly made, the entering end of the pin is highly stressed, approaching the yield point at this thinnest portion of the tapered thread. A slight inattention on the part of an operator results in overstressing or galling of the pipe. Even with proper makeup galling or permanent deformation occurs in the forward threads on the pin after a few engagement operations. Consequently there is an inherent limit to the usage of increased bearing pressure to assure a pressure seal. Even more, reliance on a tight thread engagement for sealing is essentially unreliable because of tolerances that must be accepted, thread damage and other non-uniformities. Recognition of these factors has led to the widespread usage of field instrumentation mentioned above.
The practical operating and cost requirements focus attention on crucial specific problems involved in making up secure leak-free joints under field conditions. A.P.I. pipe has a specified length and taper angle of thread on the box and pin, and includes defined acceptable tolerances for the tapers. The collar or box has inner diameter threads which taper inwardly from each end to a threaded mid-region or crest of smallest inner diameter, such that the pin can be threaded in to a penetration depth which is limited only by the forces which can be exerted during makeup. With A.P.I. pipe, assuring proper coupling of the pin to the box on the rig floor presents significant problems. The "last scratch mark" of the threaded region defines a reference against which the nominal depth of insertion can be gauged, but it is not practical to monitor last scratch position in the fast paced and environmentally imperfect conditions under which extremely long strings of production tubing and casing must be assembled. Moreover, many imperfections and defects relate to sealing problems and prevent use of pipe for this reason only even though mechanical engagement may be adequate. Under present practice such pipe can be used in only very limited ways or must be scrapped.
Details as to standard A.P.I. pipe can be found in A.P.I. Standard 5B (10th edition), March 1979 and Supplement 1 thereto issued March 1980, these documents being issued by the American Petroleum Institute, Production Department, 211 North Ervay, Suite 1700, Dallas, Tex. 75201. The standards define such factors as the angles of taper, pitch, profile, and effective thread length, as well as the plane of hand tight engagement and the plane of "vanish point" which defines a theoretical position for a power-tight makeup. The term "taper" is usually regarded as involving both the angle and the dimension of the pipe thread, which together determine the depth of penetration of the pin end into an ideal collar. In these standards, it should be noted that with round thread or buttress thread a space is necessary between mating thread profiles. In the round thread, for example, this is referred to as a "root helix", and this small clearance provides a continuous path through which leakage or buildup of a high pressure fluid can occur.
If the pin taper is at the opposite end of the tolerance limit from the box thread taper, or if the dimensional variations are at opposite tolerance limits, then adequate thread engagement may occur either well before or well after the nominal position defined by reference to the last scratch mark. The assumption that adequately firm thread engagement will provide the needed internal and external seal is not a satisfactory basis for makeup because of the problems of thread damage and pressure buildup within the root helix at the extremely high pressures that are encountered.
It should be noted, as evidenced by U.S. Pat. Nos. 2,980,451, 3,047,316, 3,054,628, 3,831,259 and 3,923,324, as well as some of the earlier referenced patents, that it has been common for a long time to utilize seal elements positioned within the threaded region of a pin and box junction.
Such seals are generally referred to in the industry as Atlas-Bradford seals, and their benefits must be weighed against their disadvantages. They are incorporated in a portion of the threaded structure, which means in turn that they reduce both the length of thread engagement and the wall diameter. Furthermore, they introduce internal stress risers in a critical portion of the thread engagement zone. In addition, the threads of an entering pin deform or tap through the seal, and it is found in practice that an imperfection or irregularity can cause the seal to become caught and either disengaged or destroyed during makeup. Such seals are also disposed against the external end of the threaded region and equally provide a barrier to the release of internal pressure. This in turn means that an internal pressure can penetrate between the opposed pin and box threads throughout most of their lengths, and can cause the differential deformation that might lead to decoupling of the string.
Other coupling designs are based on the usage of metal-to-metal seals at one or both ends of the threaded region. Since metal-to-metal seals require an almost perfect mirror finish, contact between metal surfaces does not always provide a reliable seal, particularly under field conditions, because of galling, scratches, or other damage that might occur to one or the other of the surfaces. Furthermore, when surfaces must be precisely placed with the degree of accuracy required for these seals, the product cost is increased by multiple, rather than fractional, amounts. Under actual operating conditions, penetration of a corrosive high pressure fluid into the threaded engagement region over a substantial period of time may not only have catastrophic effects, but may create time consuming problems because of corrosion of the threads, drying out of the pipe joint compound or lubricant, and weakening of the coupling. External pressures can also act adversely on the coupling system. Leakage in a casing may, for example, surround an encompassed tubing with a high pressure, low flow environment that is of substantially higher pressure than the interior of the tubing. Such pressures can build up within the threaded portion of the coupling and have the previously mentioned adverse effects.
Despite all of the efforts which have been exerted toward improvement of designs and rig floor assemblies, there still remains a need for improved couplings that can be used with existing inventories of A.P.I. pipe. Furthermore, such improved couplings should facilitate reliable and uniform pipe makeup on the rig floor and require no more time than is presently used for standard pipe makeup. There is also need for precise sealing and mechanical engagement in other contexts as well, such as pipe lines using A.P.I. couplings. In some long pipe lines threaded couplings are used because corrosive fluids require plastic coatings that would be damaged by welding temperatures. Extremely high stresses may be introduced along the pipe axis by thermal expansion and contraction, but at the same time the pressure seal must be maintained.